There have been hitherto known a photocathode used to detect light to be detected having a prescribed wavelength, and an electron tube having the photocathode. The photocathode has a light absorption layer for absorbing light having a prescribed wavelength and emitting photoelectrons. Light to be detected is made incident to the light absorption layer, and the incident light to be detected is converted to photoelectrons, whereby the light to be detected can be detected. Various kinds of semiconductor materials are used for the light absorption layer, and polycrystalline diamond has been disclosed as a material having a high quantum efficiency for photoelectrical conversion of ultraviolet light in Japanese Unexamined Patent Application Publication (Tokukai) No. H10-149761.
In connection with recent high semiconductor integration design, miniaturization of semiconductor integrated circuits has been rapidly promoted. At present, photolithography is greatly expected as a method of manufacturing minute semiconductor integrated circuits, and research into light sources is advancing from an ArF excimer laser to lasers having shorter wavelengths such as an F2 laser, etc. In connection with developments of techniques using light having shorter wavelengths such as ultraviolet light, etc., photodetectors for monitoring ultraviolet light have been required.